ci(sea): cache ~/.npm in kernel-e2e (mirror main.yml) so npm ci doesn't hit unreachable dev proxy

Two package-lock entries (flatbuffers etc.) resolve to npm-proxy.dev.databricks.com,
which the protected runner can't reach (ECONNRESET). main.yml's e2e job only
succeeds because it caches ~/.npm — those tarballs load from the warm cache, never
fetched. kernel-e2e had no such cache, so npm ci failed deterministically.

Add the identical actions/cache step (same key) before npm ci. The driver has a
single root package-lock.json and the kernel checkout adds none, so
hashFiles('**/package-lock.json') matches main.yml's key exactly → restores main's
warmed cache. Confirmed end-to-end up to this point: App token + private-kernel
checkout + JFrog now all pass; npm ci was the only remaining failure.

Co-authored-by: Isaac
Signed-off-by: Madhavendra Rathore <madhavendra.rathore@databricks.com>

Author: msrathore-db

.github/workflows/kernel-e2e.yml

@@ -281,6 +281,24 @@ jobs:
           EOF
           echo "CARGO_REGISTRIES_JFROG_TOKEN=Bearer ${JFROG_ACCESS_TOKEN}" >> "$GITHUB_ENV"
 
+      # Restore the same `~/.npm` cache main.yml's e2e job warms. A couple of
+      # lockfile entries resolve to `npm-proxy.dev.databricks.com` (a dev proxy
+      # the protected runner can't reach); they only ever load from this cache.
+      # The key matches main.yml exactly (driver has a single root
+      # package-lock.json; the kernel checkout adds none), so this restores
+      # main's warmed cache and `npm ci` never hits the unreachable proxy.
+      - name: Cache node modules
+        uses: actions/cache@0057852bfaa89a56745cba8c7296529d2fc39830 # v4
+        env:
+          cache-name: cache-node-modules
+        with:
+          path: ~/.npm
+          key: ${{ runner.os }}-build-${{ env.cache-name }}-${{ hashFiles('**/package-lock.json') }}
+          restore-keys: |
+            ${{ runner.os }}-build-${{ env.cache-name }}-
+            ${{ runner.os }}-build-
+            ${{ runner.os }}-
+
       - name: Install driver deps
         run: npm ci